On-load tap changers have been in use worldwide on a large scale for many years for uninterrupted switching between different winding taps of tapped transformers. So-called reactor switches, which are particularly widespread in North America, have a switch-over reactance that enables a slow, continuous switching. On-load tap changers according to the resistance fast switching principle usually consist of a selector for power-free selection of the respective winding tap of the tapped transformer that is to be switched to and a load changeover switch for the actual changing over from the previous to the new, preselected winding tap. The load changeover switch for that purpose usually comprises switch contacts and resistance contacts. The switch contacts in that case serve for direct connection of the respective winding tap with the load diverter and the resistance contacts for temporary connection, i.e. bridging-over by means of one or more switching-over resistances. However, developments in recent years have led away from load changeover switches with mechanical switching contacts in insulating oil. Instead, vacuum interrupters are increasingly employed as switching elements.
An on-load tap changer of that kind with vacuum interrupters is disclosed in, for example, DE 10 2009 043 171 [US 2012/0139510]. Here, a load changeover switch carries a drive shaft, which is drivable by an energy store, with at least one cam disk. The cam disk has a plurality of cams, wherein two cams arranged at the cam disk at the end have a profile that departs from the circular shape, in the form of lobes at which a respective roller that is connected with a vacuum interrupter by way of a rocker arm and that scans the profiled contour of the respective cam, is guided with maintained contact.
In one specific category of on-load tap changer, i.e. so-called load selectors, the described means for selection of a new winding tap and the means for the actual load changeover are constructionally combined and actuated in common. Tap changers according to the principle of resistance fast switching for uninterrupted switching from one winding tap at the tapped transformer to another are usually constructed so that the respective fixed tap contacts electrically connected with the output lines of the tap windings are arranged in one or more horizontal planes circularly at an insulating-material frame or cylinder and are connectable by rotatable contact bridges actuated by concentric drive shafts. In the case of load selectors in which the tap selection and the actual load switching are combined, actuation of the contact bridges in that case takes place abruptly after triggering of an energy store, usually a spring-force store, drawn up by the drive shaft of the switch.
In departure from these usual constructions, a tap changer with linear contact actuation is described in DE 42 37 165 [U.S. Pat. No. 5,523,671], wherein the fixed tap contacts extend along a track in the interior of the switch and are connectable by a displaceable switching mechanism that in turn is driven by the drive shaft. The vertically displaceable switching mechanism in that case consists of a draw-up carriage that is continuously drivable by the drive shaft and that preselects the new fixed tap contact, and a drive output part that can be drawn up by the draw-up carriage by means of an energy store and that after triggering abruptly runs after the draw-up carriage and in that case executes the actual load changeover from the previous tap of the tapped winding to the preselected new tap. The switch elements required for that purpose are a component of the output drive part.
A further on-load tap changer, which is attributable to the applicant, with linearly arranged selector contacts has become known from DE 198 47 745. Disposed above the selector contacts, thus locally separated, are the stationary vacuum interrupters, which are associated with the respective selector contacts of each phase, for the actual, abrupt load changeover. A spring energy store is necessary for actuation of this on-load tap changer that store actuates the selector contacts during its drawing up movement as well as the vacuum switching elements by its abrupt drive output movement.
Such on-load tap changers are in principle known as built-in changers or attachment changers. A typical built-in changer usually consists of a load changeover switch in a separate oil vessel for the uninterrupted load changeover and, arranged thereunder, a selector for preselection of the winding tap of the tapped transformer that is to be switched to. Such built-in changer, hence the name, is introduced from above completely into the oil-filled transformer vessel and mounted. In that case it is ensured by the separate oil vessel of the load changeover switch that the oil volume thereof does not mix with the surrounding insulating oil in the transformer vessel. It is then generally necessary to lead electrical lines as a so-called line structure from the individual winding taps in the interior of the transformer through the transformer vessel to the tap changer.
A typical attachment changer, such as is known from, for example, the company publication of Reinhausen Manufacturing Inc., ‘VACUTAP (Registered Trade Mark) RMV-A On-Load Tap Changer for Regulating Transformers’, has a separate, surrounding housing, usually of sheet metal, and is placed laterally on the outer wall of the transformer. The housing of such an attachment changer is filled with insulating oil. It is also necessary in this case to lead the electrical lines from the individual winding taps in the interior of the transformer through the entire transformer vessel and moreover additionally through a so-called transformer board, which seals and is also termed barrier board, outwardly to the laterally attached tap changer. DE 29 23 170 [U.S. Pat. No. 4,234,864] describes such a transformer board, there termed ‘terminal board’. It is necessary—because the attached tap changer is accommodated in a separate oil-filled housing—for there to be separation from the similarly oil-filled transformer vessel by an oil-tight partition wall, wherein the connections for connecting the transformer taps with the tap changer are provided at the ‘terminal board’. Not only the necessary line structure, but also the transformer board are complicated and costly.
In addition, it is necessary in all the prior art to provide a separate oil circuit for the tap changer that usually also includes a separate protective relay, individual pipelines as well as a separate expander and also separate dehumidifier.